The present invention relates to a high tension voltage source using a blocking oscillation circuit, wherein the output voltage is stabilized against the variations of the line voltage and the ambient temperature, and also the power consumption is reduced so that the heating of elements used therein is suppressed. The result is a stable and compact apparatus.
In general, high tension voltage source using a blocking oscillation circuit, a pulse transformer, and a rectification means have, as is well known, an advantage of compactness in size and light weight as for the purpose of low current capacity use, and therefore it has been widely used for xerography, cathode-ray tube applied apparatuses, and small-size electrostatic dust collectors, i.e., cottrells, etc.
For these purposes, those circuits having constructions as shown in FIG. 1, illustrating a self-exciting type, have generally been used. However, in such the construction, the output voltage changes largely due to fluctuations of the line voltage. Also, because the transistor used as a switching element in the circuit has a temperature dependent characteristic, the output voltage changes due to fluctuations of the ambient temperature, too. Describing further the prior art as set out in FIG. 1, the collector of a transistor 1 is connected to one terminal of an A.C. line source through a primary winding 3 of a step-up transformer 2 and power source rectifier diode 4. The other terminal of the A.C. line source is connected to the emitter of said transistor. The base of said transistor 1 is connected through feedback winding 5 to the series connection point of resistors 6 and 7 which supply a bias voltage through feedback winding 5 and the feedback winding 5 is wound so that positive feedback takes place. Hereupon, 8 is a smoothing capacitor, 9 is a damper diode, 10 is a secondary winding of the transformer 2, 11 is a high voltage rectifier diode, and 12 is a high voltage smoothing capacitor. In most cases, stray capacity of a connected load serves as the high voltage smoothing capacitor 12, or in some case the smoothing capacitor 12 is omitted according to the purposes. Also, A.sub.1 and A.sub.2 are terminals to be connected to the A.C. line source and B.sub.1 and B.sub.2 are high tension output terminals.
If an A.C. voltage is impressed to the terminals A.sub.1 and A.sub.2, it is rectified by the diode 4 and smoothed by the smoothing capacitor 8, and then a bias voltage devided by the bias resistors 6 and 7 is supplied to the base of the transistor 1 thereby making the transistor 1 conductive. The collector current rises according to the circuit time constant determined mainly by the effective inductance, capacitance and resistance of the primary winding 3 of the transformer. During this course of time, a positive feedback voltage is induced in a feedback winding 5, and hence the transistor 1 remains conductive.
If the collector current attains such a value that it is suppressed by the positive feedback voltage after its saturation, the transistor 1 is cut off rapidly by the action of reverse regeneration. At this time, a large inverse electromotive force appears in the primary winding 3 and hence the high voltage pulse takes place. This pulse voltage is stepped up with the secondary winding 10 and is rectified and smoothed by the diode 11 and the capacitor 12, respectively. Accordingly a D.C. high tension voltage is obtained. The voltage V of the output pulse in this circuit is given by EQU V = L di/dt ,
wherein t is time, i is a primary winding current and L is a composite inductance of the primary winding and the secondary winding of the transformer and is a fixed constant determined by the construction of the transformer. In the circuit, the cut-off time is determined by the circuit time constant and hence is a fixed constant. Therefore, neither L nor cut-off time shows large variations with temperature changes. However, since the primary windingf current i is determined by characteristics of the transistor and the source line voltage, then di/dt depends largely upon the temperature characteristics of the transistor and the source line voltage variation. Thus, in such an example prior art is defective in that the output voltage fluctuates largely depending upon the source line voltage variation and the ambient temperature, as shown by the curves of broken lines of FIG. 5. Hereupon, the parameter VB in FIG. 5 represents the input voltage.